More than one million preschoolers in the U. S. receive special education services for specific language impairment (SLI). A significant number of these children demonstrate difficulty with lexical acquisition. This deficit is particularly devastating in young children because lexical acquisition is strongly associated with later literacy development. Researchers have hypothesized that phonological memory (PM) deficits or semantic skills (SS) deficits may cause this problem; however, research to date has not investigated the joint impact of both types of deficits on lexical acquisition. To evaluate the relationships among PM skills, SS, lexical acquisition, and SLI, young children with SLI and age- and gender-matched peers with normal language (NL) will participate in a series of group studies designed to determine (1) the extent to which performance on PM tasks or lexical SS tasks contributes to accurate identification of children with SLI and predicts word-learning performance; (2) whether phonologically-based or semantically-based treatment or a combination of both improves performance on tasks designed to measure those skills; and (3) whether improved performance on PM and SS tasks relates to improved word-learning performance. Treatment efficacy studies will include children ages 3 through 6 years to determine whether the relationships among PM skills, SS, and word learning vary as a function of age. These studies will provide insight into the underlying language deficits of children with SLI, will help determine the cause of poor lexical learning, and will provide treatment efficacy data important for effective treatment of this disorder.